Antimicrobial peptides (AMPs) are a relatively newly discovered group of antimicrobial agents with new modes of action.
AMPs are widely distributed in animals, plants and microbes and are among the most ancient host defense factors. Most of the peptides are cationic and amphipatic in nature; a feature that allows interaction with the negatively charged bacterial or fungal membrane.
The peptides range in size from 6-7 amino acids up to 60. More than 500 different AMPs have been isolated to date. They can be divided into several classes based on structure or amino acid composition. The simplest structures are small alpha-helical peptides. Other AMPs folds into beta-sheet structures, while others again form rigid, disulfide bridged tertiary structures.
The AMPs are normally microbicidal (as opposed to static) and are extremely fast acting. Usually, the target organism is killed within minutes. They work by interfering with the membrane function of the target organisms. Several different mechanisms of actions have been shown to exist, but for most AMPs, the overall result is membrane disruption and/or cell lysis.
The selectivity of microbial membranes is mediated by membrane composition, membrane charge and trans-membrane potential. Microbial membranes have a higher negative charge than the membrane of higher organisms, contains different types of phospholipids, and no cholesterol.
It has proven extremely difficult to induce resistance to AMPs in target organisms. This is a reflection of the target; multiple genomic alterations would have to occur to significantly alter the membrane composition or charge.